DESCRIPTION (Taken from the application): Skeletal muscle contusion is associated with secondary tissue responses of edema and inflammation. Currently, contusions are treated with cryotherapy to reduce these secondary responses even though the mechanism of action of cryotherapy is unknown. Contusions are accompanied by histamine release which, in turn activates leucocytes, the primary cells involved in secondary intracellular responses following contusion. We hypothesize that leucocytes are activated following contusion, producing leukocyte/endothelial interactions that lead: 1) to increased microvascular permeability resulting in tissue edema and 2) to transendothelial migrations of leukocytes across the endothelial cell (EC) layer into the interstitium producing cell damage and inflammation. We will determine the role of adhesion molecules in leukocyte activation, rolling, and adhesion in-vivo in chronically instrumented rats and mice by direct observation of striated muscle microvasculature before and after mild contusion. Alterations in endothelial permeability associated with leukocyte/EC interactions also will be evaluated. A unique animal model of contusion developed in our laboratory permits microvascular measurements to be studied over several days. Specific aspects of leukocyte/EC interactions crucial for secondary damage following contusion will be determined using animals lacking specific adhesion molecules, thereby interfering with leukocyte rolling and adhesion. The therapeutic mechanisms of tissue cooling on leukocyte/EC interactions also will be studied. Since histamine from mast cells is thought to initiate leukocyte rolling and adhesion following contusion, the importance of this molecule will be studied in a transgenic mouse subgroup (deficient in mast cells) subjected to contusion. Microvascular mechanisms of secondary injury following contusion have not been studied previously. The proposed novel experiments will determine whether leukocyte/EC interactions are responsible for significant changes in endothelial permeability associated with skeletal muscle contusions and will provide insight into the development of improved therapies to reduce the morbidity of edema associated with skeletal muscle contusion.